Digital audio systems are well known in the prior art. Presently, two types of digital audio systems are generally available, the compact disc (CD) and the digital audio tape (DAT). While the advantages of digital recordings over conventional analog recordings in terms of accuracy are well-known, until recently, digital audio systems had generally failed to attract critical listeners of professional or high-end audio systems. Such listeners are accustomed to enjoying immaculately precise and realistic music reproduction currently possible with professional or high-end analog audio systems. Although digital recordings offer superior recording capabilities, various deficiencies in existing digital audio equipment have prevented the same type of reproduction of high quality realistic music from CD and DAT digital recordings as is possible with professional analog audio systems.
The goal of any digital audio system is to sample and reconstruct an analog audio signal without noticeable changes to the original audio signal so as to recreate authentic sounding music. If, for example, the original audio signal is sampled at a recording studio and the digital samples are stored on a CD, then the CD player must retrieve the digital samples and reconstruct an analog waveform in the form of an audio output signal which is as close as possible to the waveform of the original audio signal.
One of the critical steps in this reconstruction process is the digital-to-analog conversion process wherein the digital samples are converted back into an analog signal. This step of the conversion process is typically accomplished by using a digital-to-analog converter to convert a digital sample containing a predetermined number of bits of information into a high impedance analog line signal such that the amount of current in the analog line signal is proportional to the value of that digital sample as expressed by the bits of information. Many different types of digital-to-analog converters (DACs) are well known in the prior art, including colinear DACs, resistor-ladder network DACs, multiplying DACs, flash conversion DACs and single bit DACs. Once the analog line signal is created by the DACs, it is then typically sent to an analog amplifier to produce an analog power audio output signal capable of driving a low impedance speaker.
Unlike other applications which make use of DACs as part of a digital-to-analog conversion process, the use of DACs in a high performance digital audio system involves signal quality issues that are not generally present when using DACs for other applications. Examples of such signal quality issues which are unique to the audio environment include time displacement error, low level linearity and group delay distortion. Present digital audio systems have attempted to address these issues of signal quality, as well as other issues with respect to deficiencies in the stereophonic imaging capabilities of digital audio systems by utilizing a technique known as oversampling as part of the digital-to-analog conversion process. One such oversampling technique which has received critical acclaim for improving sound reproduction in existing digital audio systems is the time domain interpolation technique which the subject of previously identified U.S. patent application Ser. No. 07/268,830, filed Nov. 8, 1988, entitled TIME DOMAIN INTERPOLATION OF DIGITAL AUDIO SIGNALS, which is assigned to the assignee of the present invention.
However, even the use of such advanced oversampling techniques as time domain interpolation has not resolved all of the deficiencies inherent in using existing digital audio systems to reproduce analog audio signals. Although the oversampling process in existing digital audio systems may reproduce a relatively accurate version of the original audio signal in digital form, the music, as heard by the listener of such a digital audio system, often suffers in comparison to high end or professional analog audio system as a result of the use of analog amplifiers as the final separate stage of the digital-to-analog conversion process.
The use of analog amplifiers as a final separate stage of the digital-to-analog conversion process of existing digital audio systems effectively prevents such systems from recreating realistic high quality sound. Present digital audio systems use DACs which are only capable of generating line level analog signals (signals having power levels on the order of 1 milliwatt and designed to drive only a high impedance load). These line level analog signals are then sent through a series of analog components to provide tone control, volume control and power amplification in order to produce the analog power audio output signal which is used to drive a speaker. Regardless of how accurate the digital-to-analog conversion process is, the use of such analog pre-amplifier and amplifier components reintroduces the very type of signal coloration and distortion and signal degradation which a digital audio system is designed to avoid. In particular, the use of analog volume control and tone control techniques after the generation of the analog line signal by the DAC inherently creates signal distortion in order to accomplish volume or tone control. In addition, the use of a separate analog power amplifier in present digital audio systems subjects the audio output signal to the nonlinear amplification range of the analog power amplifier which can further distort the audio output signal, especially those portions of the audio output signal which carry the high frequency or transient images associated with sounds such as music.
Although existing digital audio systems are adequate for reproducing musical sound, it would be advantageous to have a digital audio system which is capable of direct power output digital to analog conversion of digital audio signals so as to avoid the signal coloration and distortion and signal degradation and non-linearity problems associated with the analog pre-amplification and power amplification stages of existing digital audio systems.